Method for heating exhaust

ABSTRACT

A method according to the present invention heats exhaust introduced to an exhaust emission purifier by injecting fuel into an exhaust passage through a fuel addition valve toward a heat generating portion of a glow plug disposed in the exhaust passage located upstream of the exhaust emission purifier, and then igniting the fuel. In the method, a rate of the fuel injected through the fuel addition valve which directly contacts the heat generating portion is set to between 8% and 55%.

TECHNICAL FIELD

The present invention relates to a method of heating exhaust gas to beintroduced into an exhaust gas purifying device by supplying fuel intoan exhaust passage from a fuel supply valve disposed upstream of theexhaust gas purifying device in an exhaust pipe, and then by heating andigniting the fuel.

BACKGROUND ART

In recent years, coping with strict emission standards set on internalcombustion engines has lead to a necessity for facilitating theactivation of an exhaust gas purifying device at the start of itsinternal combustion engine, maintaining its active state during theoperation of the internal combustion engine, and so on. In this respect,Patent Literature 1 and the like have proposed internal combustionengines in which an exhaust gas heating device is installed upstream ofan exhaust gas purifying device in an exhaust passage. This exhaust gasheating device generates heating gas within exhaust gas and suppliesthis generated heating gas into the exhaust gas purifying device givenat the downstream side to thereby facilitate the activation of theexhaust gas purifying device and maintain its active state. To do so,the exhaust gas heating device generally includes a fuel supply valvewhich adds fuel into the exhaust passage, and an igniter such as a glowplug which heats and ignites the fuel to generate heating gas.Meanwhile, in the conventional exhaust gas heating device disclosed inPatent Literature 1, the igniter is disposed in proximity to the fuelsupply valve. Hence, the fuel supplied into the exhaust passage from thefuel supply valve and the exhaust gas may fail to be mixed sufficiently,leading to incomplete combustion of the ignited fuel in some cases. Tosolve this, Patent Literature 1 proposes an idea that a receiving platewhich receives the fuel injected from the fuel supply valve and scattersthat fuel inside the exhaust passage is installed in the exhaustpassage.

CITATION LIST Patent Literature

-   PTL1: Japanese Patent Laid-Open No. 2010-084710

SUMMARY OF INVENTION Technical Problem

A recent study of the exhaust heating apparatus disclosed in PatentLiterature 1 confirms that fuel from a fuel addition valve impinging ona heat generating portion of ignition means is not directly ignited bythe heat generating portion. That is, the study finds that the fuelhaving impinged on the heat generating portion is heated and vaporizedby the heat generating portion and then ignited and that the firespreads to mixed gas of fuel droplets suspended around the heatgenerating portion and exhaust flowing in from an upstream side of anexhaust passage, resulting in diffusion of the combustion. Thus, whenthe fuel is injected toward the heat generating portion of the ignitionmeans so as to impinge on the heat generating portion, the heatgenerating portion may be cooled and fail to achieve appropriateignition or the fuel, even when successfully combusted, may fail todiffuse properly, depending on conditions such as the amount and rate ofthe fuel. In this case, an increased amount of unburned HC may begenerated and flow out directly toward a downstream side of the exhaustpassage, or an increased amount of smoke may be generated.

An object of the present invention is to provide a method for enablinggeneration of unburned HC and smoke to be suppressed compared to theconventional technique, if the exhaust heating apparatus is used.

The present invention is based on the present inventors' novel knowledgethat the rate of a portion of the fuel injected through the fueladdition valve which directly contacts the heat generating portion ofthe ignition means is important for generation of unburned HC and smoke.That is, a method according to the present invention of heating exhaustintroduced to an exhaust emission purifier by injecting fuel into anexhaust passage through a fuel addition valve toward a heat generatingportion of an ignition means disposed in the exhaust passage locatedupstream of the exhaust emission purifier, and then igniting the fuel,is characterized in that a rate of the fuel injected through the fueladdition valve which directly contacts the heat generating portion isset to between 8% and 55%.

Here, the following is shown in FIG. 6: the relationship between therate of a portion of the fuel injected through the fuel addition valvewhich directly contacts the heat generating portion of the ignitionmeans (this rate is hereinafter referred to as the rate of fuelimpinging on the heating portion) and the amount of increase in exhausttemperature. The relationship between the rate of fuel impinging on theheat generating portion and the amount of smoke generated per cubiccentimeter of fuel is shown in FIG. 7. As apparent from FIG. 6 and FIG.7, if the rate of fuel impinging on the heat generating portion is lowerthan 8%, the heat generating portion ignites the fuel but fails toeffectively generate mixed gas around the heat generating portion,hindering combustion from diffusing successfully. Thus, an increasedamount of unburned HC passes through the exhaust heating apparatuswithout being ignited, rapidly making the combustion improper. Incontrast, if the rate of fuel impinging on the heat generating portionis greater than 55%, the amount of smoke generated increases rapidly.

In the method for heating exhaust according to the present invention, anamount of fuel injected, during a single injection operation, throughthe fuel addition valve when the fuel is intermittently injected intothe exhaust passage, is preferably set to between 10 mm³ and 75 mm³.

Duration of injection through the fuel addition valve during a singleinjection operation is preferably set to between 1.5 ms and 15 ms.

An amount of fuel injected through the fuel addition valve per unit timeis preferably set to between 80f cc and 1,000 cc per minute.

An area of the heat generating portion which overlaps the conical fuelinjection region projected on a plane perpendicular to a center axis ofthe fuel injection region and containing an axis of the heat generatingportion may be set to between 8% and 55% of an area of the fuelinjection region.

The fuel injected though the fuel injection valve may be light oil orbiofuel.

Preferably, the heat generating portion of the ignition means is heatedand the fuel is injected into the exhaust passage through the fueladdition valve only if exhaust flowing through the exhaust passage has aflow rate of 50 grams per second or less.

Advantageous Effects of Invention

The present invention sets the rate of the fuel injected through thefuel addition valve which directly contacts the heat generating portionto between 8% and 55%. This enables sufficient mixed gas to be formedaround the heat generating portion without excessively cooling the heatgenerating portion. As a result, the amount of smoke generated can berestrained from increasing, with proper combustion constantly achieved.

If the amount of fuel injected, during a single injection operation,through the fuel addition valve when the fuel is intermittently injectedinto the exhaust passage, is set to between 10 mm³ and 75 mm³, propercombustion can be constantly and more reliably achieved, and the amountof smoke generated can be more reliably restrained from increasing.

If the duration of injection through the fuel addition valve during asingle injection operation is set to between 1.5 ms and 15 ms, the fuelcan be constantly appropriately ignited and proper combustion can beensured.

If the amount of fuel injected through the fuel addition valve per unittime is set to between 80 cc and 1,000 cc per minute, proper initialframe can be formed and an appropriate flame propagation state can bemaintained. Thus, proper combustion can be more reliably achieved.

The present invention sets the area of the heat generating portion whichoverlaps the conical fuel injection region projected on the planeperpendicular to the center axis of the fuel injection region andcontaining the axis of the heat generating portion, to between 8% and55% of an area of the fuel injection region. This allows easy setting ofa preferable relative position between the fuel addition valve and theheat generating portion of the ignition means.

Even if the fuel injected though the fuel injection valve is light oilor biofuel, ignition is unlikely to be delayed, resulting in appropriateflame propagation and constantly proper combustion.

The present invention heats the heat generating portion of the ignitionmeans and injects the fuel into the exhaust passage through the fueladdition valve only if exhaust flowing through the exhaust passage has aflow rate of 50 grams per second or less. This maintains an air-fuelratio measured around the heat generating portion in a rich state,allowing the fuel to be constantly properly ignited.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual drawing of an engine system according to anembodiment in which a method for heating exhaust according to thepresent invention is applied to a vehicle with a multicylinder internalcombustion engine;

FIG. 2 is a control block diagram of an essential part of the embodimentshown in FIG. 1;

FIG. 3 is an enlarged cross-sectional view of an essential part of anexhaust heating apparatus in the embodiment shown in FIG. 2;

FIG. 4 is a cross-sectional view taken along line IV-IV in FIG. 3;

FIG. 5 is a flowchart illustrating a control procedure for an exhaustheating process according to the present embodiment;

FIG. 6 is a graph illustrating the relationship between the rate of aportion of fuel which impinges on a heat generating portion and theamount of increase in exhaust temperature; and

FIG. 7 is a graph illustrating the relationship between the rate of aportion of the fuel which impinges on the heat generating portion andthe amount of smoke generated.

DESCRIPTION OF EMBODIMENTS

An embodiment in which a method for heating exhaust according to thepresent invention is applied to a vehicle with a compression-ignitionmulticylinder internal combustion engine mounted therein will bedescribed below in detail with reference to FIG. 1 to FIG. 5. However,the present invention is not limited to the embodiment, and theconstruction thereof may be freely modified corresponding to requiredcharacteristics. The present invention is effectively applied to a sparkignition type internal combustion engine in which gasoline, alcohol, LNG(Liquefied Natural Gas) or the like is used as fuel to be ignited by aspark plug, for example.

FIG. 1 schematically shows an essential part of an engine systemaccording to the embodiment. FIG. 2 schematically shows control blocksfor the essential part. The following are omitted from FIG. 1: a valvegear and a muffler for intake and exhaust of an engine 10, and anexhaust turbocharger, an EGR system, and the like which are commonlyused as auxiliary machines for the engine 10. Furthermore, it should benoted that some of various sensors required for smooth operation of theengine are omitted for convenience.

The engine 10 according to the present embodiment is acompression-ignition multicylinder internal combustion engine thatspontaneously ignites light oil, biofuel, or a mixture thereof as a fuelby injecting the fuel directly into a combustion chamber 10 a in acompressed state through a fuel injection valve 11. However, the engine10 may be a single cylinder engine in connection with thecharacteristics of the present invention.

A cylinder head 12 includes an intake port 12 a and an exhaust port 12 bformed therein and each located opposite a combustion chamber 10 a, andincorporates a valve gear (not shown in the drawings) including anintake valve 13 a that opens and closes the intake port 12 a and anexhaust valve 13 b that opens and closes the exhaust port 12 b. The fuelinjection valve 11 located opposite the center of an upper end of thecombustion chamber 10 a is also assembled to the cylinder head 12 so asto be sandwiched between the intake valve 13 a and the exhaust port 13b. The amount of fuel fed into the combustion chamber 10 a through thefuel injection valve 11 as well as injection timing are controlled by anECU (Electronic Control Unit) based on the operating status of thevehicle including the position of an accelerator pedal 14 pressed by adriver. The position of the pressed accelerator pedal 14 is detected byan accelerator opening sensor 16, which then outputs detectioninformation to the ECU 15.

The ECU 15 includes an operating status determination section 15 a thatdetermines the operating status of the vehicle based on information fromthe accelerator opening sensor 16 and various sensors described below, afuel injection setting section 15 b, and a fuel injection valve drivingsection 15 c. The fuel injection setting section 15 b sets the amount offuel injected through the fuel injection valve 11 and the injectiontiming based on the result of the determination by the operating statusdetermination section 15 a. The fuel injection valve driving section 15c controls operation of the fuel injection valve 11 so that an amount offuel set by the fuel injection setting section 15 b is injected throughthe fuel injection valve 11 at a set timing.

A surge tank 18 is formed in the middle of an intake pipe 17 connectedto the cylinder head 12 so as to communicate with the intake port 12 aand defining an intake passage 17 a together with the intake port 12 a.A throttle valve 20 a adapted to adjust the opening of the intakepassage 17 a via an actuator 19 is incorporated in a part of the intakepipe 17 located upstream of the surge tank 18. Furthermore, an airflowmeter 21 is attached to a part of the intake pipe 17 located upstream ofthe throttle valve 20; the airflow meter 21 detects the flow rate Q_(A)of intake air flowing through the intake passage 17 a, and outputs theflow rate Q_(A) to the ECU 15. Instead of the airflow meter 21, anexhaust flow rate sensor with the same configuration as that of theairflow meter 21 may be attached to a part of an exhaust pipe 23positioned between an exhaust heating apparatus 22 described below andthe exhaust port 12 b of the cylinder head 12.

The ECU 15 further includes a throttle opening setting section 15 d andan actuator driving section 15 e. The throttle opening setting section15 d not only sets the position of the pressed accelerator pedal 14 butalso sets the opening of the throttle valve 20 based on the result ofthe determination by the operating status determination section 15 a.The actuator driving section 15 e controls operation of the throttleactuator 19 so as to open the throttle valve 20 to the degree set by thethrottle opening setting section 15 d.

A cylinder block 24 in which a piston 24 a reciprocates includes a crankangle sensor 25 attached to the cylinder block 24 and which detects andoutputs a rotation phase of a crank shaft 24 c with the piston 24 aconnected thereto via a connecting rod 24 b, that is, a crank angle, tothe ECU 15. The operating status determination section 15 a of the ECU15 determines the rotation phase of the crank shaft 24 a, the number ofrotations of the engine, the driving speed of the vehicle, and the likein real time based on the information from the crank angle sensor 25.

The exhaust pipe 23 connected to the cylinder head 12 so as tocommunicate with the exhaust port 12 b defines an exhaust passage 23 atogether with the exhaust port 12 b. An exhaust emission purifier 26adapted to detoxify harmful substances generated by combustion of mixedair in the combustion chamber 10 a is mounted in the middle of a part ofthe exhaust pipe 23 located upstream of the muffler (not shown in thedrawings) attached to a downstream end side. The exhaust emissionpurifier 26 according to the present embodiment includes at least anoxidation catalyst but may incorporate a DPF (Diesel ParticulateFilter), a NO_(x) storage catalyst, and the like. The oxidation catalystis adapted to oxidize, that is, combust unburned HC and the like mainlycontained in exhaust.

The exhaust heating apparatus 22 is disposed in the middle of a part ofthe exhaust pipe 23 located upstream of the exhaust emission purifier26; the exhaust heating apparatus 22 is adapted to generate heating gasand to feed the heating gas to the exhaust emission purifier 26 disposeddownstream of the exhaust heating apparatus 22 in order to activate theoxidation catalyst and keep the oxidation catalyst active. FIG. 3 showan extracted and enlarged essential part of the exhaust heatingapparatus 22. FIG. 4 shows an enlarged cross-sectional structure of theessential part; this cross-sectional view is taken along line IV-IV inFIG. 3. According to the present embodiment, the exhaust heatingapparatus 22 includes a fuel addition valve 27 and a glow plug 28.

The fuel addition valve 27 has the same basic configuration as that ofthe ordinary fuel injection valve 11 and is adapted to be able to feedany amount of fuel in pulse form into the exhaust passage 23 a at anytime intervals by controlling time for energization. In this case, theamount of fuel injected through the fuel addition valve during a singleinjection operation is set to between 10 mm³ and 75 mm³ when the fuel isintermittently fed into the exhaust passage. The duration of injectionduring a single injection operation is set to between 1.5 ms and 15 ms.If the amount of fuel injected during a single injection operation isless than 10 mm³ or the duration of injection is shorter than 1.5 ms,the air-fuel ratio measured around the heat generating portion 28 a ofthe glow plug 28 becomes excessively lean, leading to degradedcombustion diffusion performance. In contrast, if the amount of fuelinjected during a single injection operation is more than 75 mm³ or theduration of injection is longer than 15 ms, the latent heat ofvaporization of the fuel reduces the surface temperature of the heatgenerating portion 28 a, leading to improper combustion and an increasedamount of smoke generated. As a result, emission is deteriorated.

The fuel can be constantly properly ignited by setting the amount offuel injected through the fuel addition valve during a single injectionoperation to between 10 mm³ and 75 mm³ and setting the duration ofinjection to between 1.5 ms and 15 ms. This also allows propercombustion to be constantly and more reliably achieved and enables anincrease in the amount of smoke generated to be reliably suppressed.

The amount of fuel fed into the exhaust passage 23 a through the fueladdition valve 27 is set by a fuel addition setting section 15 f of theECU 15 based on the operating status of the vehicle including the amountof intake air and air/fuel ratio detected by the airflow meter 21.However, the amount of fuel injected through the fuel addition valve 27per unit time is set to between 80 cc and 1,000 cc per minute. If theamount of fuel injected through the fuel addition valve 27 per unit timeis less than 80 cc, the air-fuel ratio measured around the heatgenerating portion 28 a of the glow plug 28 becomes excessively lean,leading to degraded combustion diffusion performance. In contrast, ifthe amount of fuel injected through the fuel addition valve 27 per unittime is more than 1,000 cc, the latent heat of vaporization of the fuelreduces the surface temperature of the heat generating portion 28 a,leading to improper combustion and an increased amount of smokegenerated. As a result, emission is deteriorated.

Thus, proper initial frame can be formed and an appropriate flamepropagation state can be maintained by setting the amount of fuelinjected through the fuel addition valve 27 per unit time to between 80cc and 1,000 cc per minute. As a result, proper combustion can be morereliably achieved.

A fuel addition valve driving section 15 g of the ECU 15 controlsoperation of the fuel addition valve 27 so that an amount of fuel set bythe fuel addition setting section 15 f is injected through the fueladdition valve 27 at a set period.

The glow plug 28 serving as ignition means according to the presentinvention is connected to an onboard power source (not shown in thedrawings) via a glow plug driving section 15 h of the ECU 15 serving asan on/off switch. Thus, the glow plug 28 is controllably switchedbetween an energized state and a non-energized state by the glow plugdriving section 15 h of the ECU 15 in accordance with a preset program.The glow plug 28 includes the heat generating portion 28 a adapted toignite fuel injected into the exhaust passage 23 a. The heat generatingportion 28 a is disposed in the exhaust passage 23 a so as to lie in aninjection region Z_(F) for the fuel injected through the fuel additionvalve 27 to diffuse in conical form. In this case, the area (shaded inFIG. 4) of a part of the heat generating portion 28 a which overlaps theinjection region Z_(F) projected on a plane perpendicular to a centeraxis C_(F) of the injection region Z_(F) and which contains an axisC_(H) of the heat generating portion 28 a is set to between 8% and 55%of the area (shown by a circular dashed line in FIG. 4) of the fuelinjection region Z_(F). If the rate of fuel directly contacting the heatgenerating portion 28 a of the glow plug 28 is less than 8%, even thoughthe fuel impinging on the heat generating portion 28 a is ignited, aflame is restrained from diffusing and is lost because the oxygen in theexhaust around the heat generating portion 28 a has a low concentration.When the rate is maintained at 8% or more, the combustion can becontinued by propagation of the flame to the surrounding fuel. On theother hand, if the rate of fuel directly contacting the heat generatingportion 28 a of the glow plug 28 is more than 55%, droplets of the fuelwith large particle sizes reside in the flame, leading to asignificantly increased amount of smoke generated. Setting the rate toat most 55% allows the amount of smoke generated to be restrained fromincreasing.

Thus, setting the area of the heat generating portion 28 a to between 8%and 55% of the area of the injection region Z_(F) enables sufficientair-fuel mixture to be formed around the heat generating portion 28 awithout excessively cooling the heat generating portion 28 a. As aresult, the amount of smoke generated can be restrained from increasing,with proper combustion constantly achieved.

Such an impinging plate as disclosed in Patent Literature 1 can bedisposed downstream of the heat generating portion 28 a of the glow plug28 in a direction in which the fuel is injected through the fuelinjection valve 27. The presence of the impinging plate enables the fuelfed through the fuel injection valve 26 to be received to promoteatomization and flying toward the glow plug 28.

A part of the exhaust passage 23 a located on an exit side of theexhaust emission purifier 26 incorporates a catalytic convertertemperature sensor 29 that detects the temperature of exhaust exitingthe exhaust emission purifier 26 (this temperature is hereinafterreferred to as a catalyst temperature). Furthermore, an exhausttemperature sensor 30 is attached to a part of the exhaust pipe 23located upstream of the exhaust emission purifier 26 and downstream ofthe exhaust heating apparatus 22. The exhaust temperature sensor 30detects the temperature T_(E) of exhaust flowing through the exhaustpassage 23 a immediately before the exhaust flows into the exhaustemission purifier 26. The exhaust temperature sensor 30 then outputs thedetection information to the ECU 13.

The operating status determination section 15 a of the ECU 13 accordingto the present embodiment determines whether the exhaust heatingapparatus 22 needs to be actuated, that is, whether or not fuel additionis required, based on the information from the catalytic convertertemperature sensor 29 and the exhaust temperature sensor 30. Normally,the lowest temperature at which the oxidation catalyst can maintain itsactive state is defined as a reference, and the need for fuel additionis determined if the catalyst temperature is lower than the reference oris expected to be lower than the reference. However, any otherconventional well-known method for determination may be adopted asnecessary.

According to the present embodiment, while the engine 10 is in amotoring state, that is, the engine 10 is in operation, if the openingof the accelerator pedal 14 becomes zero to bring the engine 10 into afuel cut state in which no fuel is injected through the fuel injectionvalve 11, an exhaust heating process is carried out as necessary. Thatis, upon determining that the oxidation catalyst needs to be activatedor kept active, the apparatus injects fuel through the fuel injectionvalve 27 and heats exhaust flowing through the exhaust passage 23 a.Thus, when the engine 10 is in the fuel cut state, fuel is injected intothe combustion chamber 30 through the fuel addition valve via a fuelinjection chamber 31, thus increasing the temperature of exhaust flowingthrough the exhaust passage 23 a. However, if the exhaust flowingthrough the exhaust passage 23 a has a flow rate Q_(A) of 50 grams persecond, even when the fuel is partly ignited by the heat generatingportion 28 a, the fire is likely to be lost, making continuouscombustion of the fuel difficult. Consequently, the exhaust heatingprocess is not carried out.

FIG. 5 shows the exhaust heating process. That is, step S1 determineswhether or not fuel addition is required. Here, if the apparatusdetermines that fuel addition is required, that is, the exhaust emissionpurifier 26 needs to be activated, the processing shifts to step S2 todetermine whether or not an intake flow rate Q_(A) is 50 grams persecond or less. Here, if the processing determines that the intake flowrate Q_(A) is 50 grams per second or less, that is, execution of theexhaust heating process poses no problem, the processing shifts to stepS3 to carry out the exhaust heating process. Then, fuel is injected intothe exhaust passage 23 a through the fuel addition valve 27 and ignitedand combusted by the heat generating portion 28 a of the glow plug 28 toincrease the temperature of exhaust flowing through the exhaust passage23 a.

On the other hand, upon determining in step S1 that fuel addition is notrequired, the processing performs no operation and repeats thedetermination. Furthermore, upon determining in step S2 that the intakeflow rate Q_(A) is more than 50 grams per second, that is, avoiding theexhaust heating process is preferred, the processing also performs nooperation and returns to step S1.

It should be noted that, the present invention should be interpretedbased only upon the matters described in claims, and in theaforementioned embodiments, all changes and modifications includedwithin the spirit of the present invention can be made other than thedescribed matters. That is, all the matters in the described embodimentsare made not to limit the present invention, but can be arbitrarilychanged according to the application, the object and the like, includingevery construction having no direct relation to the present invention.

REFERENCE SIGNS LIST

-   10 ENGINE-   10 a COMBUSTION CHAMBER-   11 FUEL INJECTION VALVE-   12 CYLINDER HEAD-   12 a INTAKE PORT-   12 b EXHAUST PORT-   13 a INTAKE VALVE-   13 b EXHAUST VALVE-   14 ACCELERATOR PEDAL-   15 ECU-   15 a OPERATING STATUS DETERMINATION SECTION-   15 b FUEL INJECTION SETTING SECTION-   15 c FUEL INJECTION VALVE DRIVING SECTION-   15 d THROTTLE OPENING SETTING SECTION-   15 e ACTUATOR DRIVING SECTION-   15 f FUEL ADDITION SETTING SECTION-   15 g FUEL INJECTION VALVE DRIVING SECTION-   15 h GLOW PLUG DRIVING SECTION-   16 ACCELERATOR OPENING SENSOR-   17 INTAKE PIPE-   17 a INTAKE PASSAGE-   18 SURGE TANK-   19 THROTTLE ACTUATOR-   20 THROTTLE VALVE-   21 AIRFLOW METER-   22 EXHAUST HEATING APPARATUS-   23 EXHAUST PIPE-   23 a EXHAUST PASSAGE-   24 CYLINDER BLOCK-   24 a PISTON-   24 b CONNECTING ROD-   24 c CRANK SHAFT-   25 CRANK ANGLE SENSOR-   26 EXHAUST EMISSION PURIFIER-   27 FUEL ADDITION VALVE-   28 GLOW PLUG-   28 a HEAT GENERATING PORTION-   29 CATALYTIC CONVERTER TEMPERATURE SENSOR-   30 EXHAUST TEMPERATURE SENSOR-   C_(F) CENTER AXIS OF INJECTION REGION-   C_(H) AXIS OF HEAT GENERATING PORTION-   Q_(A) INTAKE FLOW RATE-   T_(E) EXHAUST TEMPERATURE-   Z_(F) FUEL INJECTION REGION

1. A method for heating exhaust introduced to an exhaust emissionpurifier by injecting fuel into an exhaust passage through a fueladdition valve toward a heat generating portion of an ignition devicedisposed in the exhaust passage located upstream of the exhaust emissionpurifier, and then igniting the fuel, the method comprising: setting arate of the fuel injected through the fuel addition valve which directlycontacts the heat generating portion to between 8% and 55%.
 2. Themethod for heating exhaust as claimed in claim 1, wherein an amount offuel injected, during a single injection operation, through the fueladdition valve when the fuel is intermittently injected into the exhaustpassage, is set to between 10 mm³ and 75 mm³.
 3. The method for heatingexhaust as claimed in claim 2, wherein duration of injection through thefuel addition valve during a single injection operation is set tobetween 1.5 ms and 15 ms.
 4. The method for heating exhaust as claimedin claim 1, wherein an amount of fuel injected through the fuel additionvalve per unit time is set to between 80 cc and 1,000 cc per minute. 5.The method for heating exhaust as claimed in claim 1, wherein the fuelinjected through the fuel addition valve diffuses in conical form, andan area of the heat generating portion which overlaps the conical fuelinjection region projected on a plane perpendicular to a center axis ofthe fuel injection region and containing an axis of the heat generatingportion is set to between 8% and 55% of an area of the fuel injectionregion.
 6. The method for heating exhaust as claimed in claim 1, whereinthe fuel injected though the fuel injection valve is light oil orbiofuel.
 7. The method for heating exhaust as claimed in claim 1,wherein the heat generating portion of the ignition device is heated andthe fuel is injected into the exhaust passage through the fuel additionvalve only if exhaust flowing through the exhaust passage has a flowrate of 50 grams per second or less.
 8. The method for heating exhaustas claimed in claim 5, wherein the heat generating portion of theignition device is heated and the fuel is injected into the exhaustpassage through the fuel addition valve only if exhaust flowing throughthe exhaust passage has a flow rate of 50 grams per second or less.